When signals sent from a sensor are multiplexed and collected, insulation is made between the sensors and between the sensor and a multiplexer winding ground, and effects of a potential difference therebetween are removed. Therefore, it is also required to insulate a power supply for detecting disconnection of those sensors or converting a resistance value into an analog voltage.
For example, Patent Document 1 describes a method in which, in order to detect disconnection of a thermocouple, a power supply which is generated via an insulation transformer is provided and the power supply is connected to an input end of the thermocouple through high resistance, and an input is made open and a large voltage is applied when the thermocouple is disconnected, to thereby detect the disconnection.
A conventional analog multiplexer is described with reference to FIG. 11 to FIG. 13. FIG. 11 is a diagram illustrating a configuration of a conventional analog input device. FIG. 12 is a diagram illustrating a configuration of the conventional analog multiplexer. Further, FIG. 13 are timing charts illustrating an operation of the conventional analog multiplexer.
In FIG. 11, the conventional analog input device includes a plurality of analog multiplexers 10 respectively connected to analog signal inputs Va (Va1, Va2, Va3, . . . , and Van), a plurality of amplifiers 20, a plurality of switches 30, a sample and hold circuit 40, an AD converter 50, and a drive pulse generation circuit 60 (not shown).
In FIG. 12, the conventional analog multiplexer 10 includes: a continuous pulse generation circuit 11; a buffer 14; a rectifying/smoothing circuit 15; a power supply transformer for multiplexer T1 in which a resistor R1 is connected to a primary winding thereof and the rectifying/smoothing circuit 15 is connected to a secondary winding thereof; an analog signal transformer T2 in which the analog signal input Va is connected to a primary winding thereof and the amplifier 20 is connected to a secondary winding thereof; a drive transformer T3 in which a resistor R2 is connected to a primary winding thereof and a resistor R5 is connected to a secondary winding thereof; a field effect transistor FET1 connected to the secondary winding of the drive transformer T3; a field effect transistor FET2 connected to the drive pulse generation circuit 60; and resistors R3 and R4 connected to a power winding of the rectifying/smoothing circuit 15.
Insulation between the analog signal and the ground is performed by the respective transformers T1, T2, and T3.
Next, the operation of the conventional analog multiplexer is described with reference to the drawings.
In FIG. 12, the field effect transistor FET2 is switched on in response to a drive pulse (see FIG. 13(a)) generated in the drive pulse generation circuit 60 to drive the drive transformer T3, and the field effect transistor FET1 connected to the secondary winding of the drive transformer T3 is switched on. Accordingly, a pulse having an amplitude in accordance with the analog signal input appears in the secondary winding of the analog signal transformer T2.
This pulse is amplified by the amplifier 20, and the amplified pulse is sampled and held at a predetermined timing by the sample and hold circuit 40 to be input to the AD converter 50, whereby analog data can be collected.
The continuous pulse generation circuit 11 is connected to the primary winding of the power supply transformer for multiplexer T1 via the buffer 14, and a pulse train for power supply (continuous pulse) appears in the secondary winding of the power supply transformer for multiplexer T1. The pulse is rectified and smoothed by the rectifying/smoothing circuit 15, and thus a direct current voltage (see FIG. 13(c)) is obtained. The direct current voltage is supplied to both ends of the analog signal input via the resistors R3 and R4 having high resistance.
When the analog signal input is not disconnected, an analog signal Va is input as it is without being affected by the direct current voltage because an analog signal source resistance is small. On the other hand, when the analog signal input is disconnected, the signal source is made open and the direct current voltage is input via the resistors R3 and R4 having high resistance. Then, a pulse voltage which is larger than the analog signal input appears in the secondary winding of the analog signal transformer T2, whereby disconnection thereof can be detected. FIG. 13 illustrate this operation by way of a timing chart.
Patent Document 1: JP 63-113622 A